DC-to-DC switching power converters are often used when a system needing a regulated power supply is powered from an unregulated DC source of power, or a regulated DC source of power at a different voltage. Typically, switching power converters are used when either the output voltage is higher than the input voltage, or the input voltage is higher than the output voltage, but not both. However, some applications require a switching power converter that can supply an output voltage that may be either higher or lower than an input voltage. One example is a battery powered system needing a regulated supply voltage at 3 volts DC. The battery supplying the battery powered system may be a lithium battery having an output voltage that varies from 3.3 volts DC down to 2.5 volts DC. A switching power converter that has an output voltage higher than the input voltage may be referred to as a boost converter. A switching power converter that has an output voltage lower than the input voltage may be referred to as a buck converter. By cascading a buck converter and a boost converter, a switching power converter can be created that can supply an output voltage that may be either higher or lower than an input voltage.
A switching power converter typically operates with a switching frequency. During each cycle of the switching frequency, an energy storage element, such as an inductor, is switched to receive power from an input power source for one portion of the cycle. For another portion of the cycle, the energy storage element is switched to provide some of its stored energy to downstream circuitry. The percentage of the cycle that the energy storage element is switched to receive power is known as the duty-cycle, which is varied to regulate the output of the switching power converter.
A switching power converter that supports both a boost mode of operation and a buck mode of operation may be arranged with a DC power source feeding a buck power converter, which feeds a boost power converter that provides a DC output. Alternatively, the switching power converter may be arranged with the DC power source feeding the boost power converter, which feeds the buck power converter that provides the DC output. This alternate arrangement may require two energy storage elements, as claimed in U.S. Pat. No. 6,348,781; however, an arrangement using a single energy storage element may provide a simpler, smaller, and more energy efficient solution. Existing designs may use separate timing and control circuitry for buck circuitry and boost circuitry; therefore, one problem facing such a switching power converter is operating when the input voltage is nearly equal to the output voltage. Smooth transitioning between boosting and bucking may be unattainable with some designs. If both the boost power converter and the buck power converter are active at the same time, unstable, inefficient, or noisy behaviors may result. Thus, there is a need for a switching power converter that supports both a boost mode of operation and a buck mode of operation, uses one energy storage element, transitions smoothly between boosting and bucking, and avoids simultaneous boosting and bucking.